Modifications of auditory feedback and its effects on the voice of adult subjects: a scoping review

ABSTRACT Introduction The auditory perception of voice and its production involve auditory feedback, kinesthetic cues and the feedforward system that produce different effects for the voice. The Lombard, Sidetone and Pitch-Shift-Reflex effects are the most studied. The mapping of scientific experiments on changes in auditory feedback for voice motor control makes it possible to examine the existing literature on the phenomenon and may contribute to voice training or therapies. Purpose To map experiments and research results with manipulation of auditory feedback for voice motor control in adults. Method Scope review following the Checklist Preferred Reporting Items for Systematic reviews and Meta-Analyses extension (PRISMA-ScR) to answer the question: “What are the investigation methods and main research findings on the manipulation of auditory feedback in voice self-monitoring of adults?”. The search protocol was based on the Population, Concept, and Context (PCC) mnemonic strategy, in which the population is adult individuals, the concept is the manipulation of auditory feedback and the context is on motor voice control. Articles were searched in the databases: BVS/Virtual Health Library, MEDLINE/Medical Literature Analysis and Retrieval System online, COCHRANE, CINAHL/Cumulative Index to Nursing and Allied Health Literature, SCOPUS and WEB OF SCIENCE. Results 60 articles were found, 19 on the Lombard Effect, 25 on the Pitch-shift-reflex effect, 12 on the Sidetone effect and four on the Sidetone/Lombard effect. The studies are in agreement that the insertion of a noise that masks the auditory feedback causes an increase in the individual's speech intensity and that the amplification of the auditory feedback promotes the reduction of the sound pressure level in the voice production. A reflex response to the change in pitch is observed in the auditory feedback, however, with particular characteristics in each study. Conclusion The material and method of the experiments are different, there are no standardizations in the tasks, the samples are varied and often reduced. The methodological diversity makes it difficult to generalize the results. The main findings of research on auditory feedback on voice motor control confirm that in the suppression of auditory feedback, the individual tends to increase the intensity of the voice. In auditory feedback amplification, the individual decreases the intensity and has greater control over the fundamental frequency, and in frequency manipulations, the individual tends to correct the manipulation. The few studies with dysphonic individuals show that they behave differently from non-dysphonic individuals.

Auditory feedback is the hearing perception of one's own voice in real time, enabling the person to monitor its intensity, frequency, and quality (5,(8)(9)(10) .Somatosensory feedback is the perception of adaptations and the motor adjustments of structures involved in the phonating process (11) .
Auditory and somatosensory feedback help produce internal references for speech motor planning and update these adjustments for the feedforward system (9) -which is theoretically described as a cortical system located in the left brain hemisphere.It is responsible for mapping the articulation movements of the lips, mandible, tongue, and larynx and stores these speech-motor adjustments based on motor, somatosensory, and auditory references (8) .The feedforward system uses these previously acquired internal references to control the voice (7) .
Different types of auditory feedback manipulations produce distinct effects on the person's voice -of which the most studied ones are the Lombard effect, the sidetone or amplification effect (12)(13)(14) , and the pitch-shift effect (15)(16)(17)(18) .
The Lombard effect occurs when the intensity of voice production increases by inserting an intense noise, which masks the auditory feedback.The voice's sound pressure is increased unconsciously and instantaneously, and when the noise is removed, vocally healthy people tend to return to the speech intensity level they were using before the noise was inserted (2,18) .
The sidetone effect is the amplification of the sound feedback, increasing the person's perception of their own voice (12) .In response to this manipulation, vocally healthy people reduce their voice's sound pressure level (12) .Hence, this effect reduces the sound pressure of the voices of patients with hyperfunctional dysphonia and creates or increases the subject's auditory perception regarding parameters and changes in their voice (8) , making it possible to monitor its fundamental frequency, quality, and intensity (8,(12)(13)(14) .
The pitch-shift effect occurs when a person with no vocal changes is auditorily exposed to changes in their own voice's frequency.This manipulation causes a reflex correction, which is known to change the frequency most commonly in the opposite direction of the manipulation.Another less often possibility is to change it in the same direction of the manipulation (10,(15)(16)(17) .
It is important to understand how vocal changes produced by different auditory feedback manipulations can be used in voice therapy and training.Little is known about experiments regarding the time of exposure to feedback, the level of amplification or masking noise, and the effectiveness of these manipulations for the voice of individuals with and without vocal complaints, whether they are occupational voice users or not.It is believed that mapping the literature on the topic through a scoping review will help examine the current scientific literature on the phenomenon and verify its gaps, envisioning future possibilities for further studies.
Given the above, the following question was raised: "What are the investigation methods and main findings in research on auditory feedback manipulation in adults' self-monitoring of voice?".

METHOD
This scoping review followed the detailed checklist of the Preferred Reporting Items for Systematic Reviews and Meta-Analyses -Extension for Scoping Reviews (PRISMA-ScR) in the Joanna Briggs Institute Reviewers' Manual (19) .This checklist has 22 items that guide the writing of the scoping review report.The review was conducted between November 2021 and November 2022.The review protocol was registered in the Open Science Framework (OSF) on November 29, 2021, under DOI:10.17605/OSF.IO/CYM9N.
Two researchers selected, included, and extracted data independently regarding study eligibility for selection and inclusion.Inclusion and exclusion criteria were defined as follows, according to PRISMA-ScR: Inclusion criteria: articles with the term "auditory feedback" in their titles and abstracts; articles addressing the topic implicit in their abstracts, related to speech motor control in adults, whether or not occupational voice users; addressing voice treatment or training; articles written in Portuguese, English, or Spanish, with no restriction on the year.These criteria were established to screen all literature available on the topic.
The exclusion criteria were articles using auditory feedback in contexts other than the subjects' own voice sound amplification (sidetone effect), auditory feedback suppression (Lombard effect), or voice frequency manipulation (pitch-shift effect); articles using feedback with external target sounds; and articles whose populations had neurological conditions.Articles whose topic was unclear in their titles or abstracts were assessed in full texts.In cases of divergence, the reviewers analyzed the articles together to define by consensus whether to keep or remove them.

Search strategy -Research question and search criteria
The review question was developed based on the PCC mnemonic strategy, corresponding to Population, Concept, and Context (19,22) .In this research, the population referred to adults; concept, to auditory feedback manipulation; and context, to speech motor control.
The sources of evidence were scientific studies published up until November 2022, addressing the use of auditory feedback, including every type of evidence.The articles were searched in the following databases: VHL (Virtual Health Library), MEDLINE (Medical Literature Analysis and Retrieval System Online), Cochrane, CINAHL (Cumulative Index to Nursing and Allied Health Literature), Scopus, and Web of Science.The DeCS/MeSH descriptors used for the concept of "auditory feedback" were Auditory Perception, Audio Feedback; and the keywords were Auditory Feedback, Auditory, External Auditory, Internal Auditory, Kinesthetic, Portable Amplification, Sound Amplification, Auditory Masking, Auditory Self-Monitoring, Sidetone, Pitch-Shift Auditory, Frequency, Shifted, Lombard Effect.As for the concept of "speech motor control", the descriptors were Voice Quality, Voice Training, Voice; and the keywords were Voice Motor Control, Voice Control, Pitch Control, Control of Voice Intensity.
Figure 1 shows the PCC search strategy developed by the lead researcher and reviewed by the second researcher.After the search, the references in the retrieved articles were also manually searched to complement the data.

Mapping data
After selecting the studies, they were exported to the Rayyan platform -Intelligent Systematic Review.They were blindly selected by each researcher, and the inclusion of conflicting studies was defined by consensus among researchers.After this selection, the form was discussed and updated according to the data they considered relevant.The studies were grouped per type of auditory feedback manipulation, observing their designs, populations, tasks used in experiments, and the dependent variables used to measure the results.Duplicates were removed, and data were extracted and organized in spreadsheets, according to the type of feedback, with the study information relevant to this review: author, country, year, objectives, method, main results, and conclusion.

Selecting evidence
After categorizing the studies, their results were summarized in two charts to make information comparison easier.

Use
(humans OR adults) AND (" feedback auditivo " OR "Auditory External" OR "Internal auditory " OR "Kinesthetic " OR "Portable amplification" OR "sound amplification" OR "Auditory masking" OR "Auditory self-monitoring" OR side tone OR "pitch-shift" OR "frequency shifted " OR "Lombard Effect") AND ("Voice Control" OR "Pitch Control" OR "Control of voice intensity" OR "Treinamento da Voz" OR "Voice Training" OR "Qualidade da Voz" OR "Voice Quality" OR "Calidad de la Voz" OR "Qualite de la voix" OR "Qualidade Vocal" OR "Treinamento da Voz" OR "Voice Training" OR "Entrenamiento de la Voz" OR "Education de la voix" OR "Controle motor da voz" OR "Voice Control") Database Strategy VHL* (humans OR adults) AND (" feedback auditivo " OR "Auditory External" OR "Internal auditory " OR "Kinesthetic " OR "Portable amplification" OR "sound amplification" OR "Auditory masking" OR "Auditory self-monitoring" OR side tone OR "pitch-shift" OR "frequency shifted " OR "Lombard Effect") AND ("Voice Control" OR "Pitch Control" OR "Control of voice intensity" OR "Treinamento da Voz" OR "Voice Training" OR "Qualidade da Voz" OR "Voice Quality" OR "Calidad de la Voz" OR "Qualite de la voix" OR "Qualidade Vocal" OR "Treinamento da Voz" OR "Voice Training" OR "Entrenamiento de la Voz" OR "Education de la voix" OR "Controle motor da voz" OR "Voice Control") MEDLINE via PubMed (humans OR adults) AND ( "Auditory External" OR "Internal auditory " OR "Kinesthetic " OR "Portable amplification" OR "sound amplification" OR "Auditory masking" OR "Auditory self-monitoring" OR "side tone" OR "pitch-shift" OR "frequency shifted " OR "Lombard Effect") AND ("Voice Control" OR "Pitch Control" OR "Control of voice intensity" OR "Voice Training" OR "Voice Quality" OR "Voice Training" OR "Voice Control") Cochrane (via Capes Portal) (humans OR adults) AND ( "Auditory External" OR "Internal auditory " OR "Kinesthetic " OR "Portable amplification" OR "sound amplification" OR "Auditory masking" OR "Auditory self-monitoring" OR "side tone" OR "pitch-shift" OR "frequency shifted " OR "Lombard Effect") AND ("Voice Control" OR "Pitch Control" OR "Control of voice intensity" OR "Voice Training" OR "Voice Quality" OR "Voice Training" OR "Voice Control") CINAHL (via Capes Portal) (humans OR adults) AND ( "Auditory External" OR "Internal auditory " OR "Kinesthetic " OR "Portable amplification" OR "sound amplification" OR "Auditory masking" OR "Auditory self-monitoring" OR "side tone" OR "pitch-shift" OR "frequency shifted " OR "Lombard Effect") AND ("Voice Control" OR "Pitch Control" OR "Control of voice intensity" OR "Voice Training" OR "Voice Quality" OR "Voice Training" OR "Voice Control") Scopus (via Capes Portal) (humans OR adults) AND ( "Auditory External" OR "Internal auditory " OR "Kinesthetic " OR "Portable amplification" OR "sound amplification" OR "Auditory masking" OR "Auditory self-monitoring" OR "side tone" OR "pitch-shift" OR "frequency shifted " OR "Lombard Effect") AND ("Voice Control" OR "Pitch Control" OR "Control of voice intensity" OR "Voice Training" OR "Voice Quality" OR "Voice Training" OR "Voice Control") Web of Science (via Capes Portal) (humans OR adults) AND ( "Auditory External" OR "Internal auditory " OR "Kinesthetic " OR "Portable amplification" OR "sound amplification" OR "Auditory masking" OR "Auditory self-monitoring" OR "side tone" OR "pitch-shift" OR "frequency shifted " OR "Lombard Effect") AND ("Voice Control" OR "Pitch Control" OR "Control of voice intensity" OR "Voice Training" OR "Voice Quality" OR "Voice Training" OR "Voice Control")

RESULTS
The synthesis of the results of the identification, selection, eligibility, and inclusion phases is described in the organogram shown in Figure 2.
The search found 19 studies (corresponding to 31.65%) that used the Lombard effect, all of them cross-sectional.Most samples comprised both male and female participants (n = 15), non-occupational voice users (n = 16), and without vocal complaints (n = 15).Only one study included individuals with dysphonia, and five studies approached singers.The tasks mainly used vowels, followed by singing and then reading.The inserted masking noise ranged from 50 to 105 dB, as there is no consensus or standardization for noise measure.Acoustic measures were the most studied outcomes, especially F0.Results agree that inserting noise to mask auditory feedback increases the person's speech intensity.Articles that researched adult singers agree that auditory feedback, even in different proportions according to the training level, contributes to tuning precision in singing.Individuals with voice problems seemingly behave differently, having difficulties in returning to the usual adjustment after being exposed to the masking noise.
Also, 12 articles (20% of those found) researched the sidetone (amplification) effect in auditory feedback (Chart 1) -11 are cross-sectional experimental studies, and the other one is a randomized clinical trial.Most studies focused on the effects of amplification on the human voice.Most samples had both male and female (n = 6) occupational voice users (n = 7), most of them teachers/professors (n = 6).Three studies approached dysphonic subjects, and one had participants with and without vocal complaints.The most used tasks were spontaneous speech (n = 8), followed by text reading (n = 3).Only one study used singing samples.The intensity of voice amplification was controlled in some studies, while other ones did not control it.Acoustic analysis was the most studied outcome, focusing on changes in sound pressure and the subjects' self-perception.The studies concordantly found decreased sound pressure levels when the auditory feedback is amplified and positive voice production effects when it was amplified in the study samples.
Four articles (corresponding to 6.66% of those found) studied the effects of manipulating the intensity, including the Lombard and sidetone effects in the same research; they were cross-sectional experimental studies.The studies used different tasks to verify the effects of amplifying the voice.The populations comprised singers (n = 2) and individuals without vocal complaints (n = 2) of both sexes (n = 4).They used reading (n = 1), speech (n = 1), and singing samples (n = 2).The intensity was manipulated with different resources, including electronic amplifiers, acoustic reflection boards, and feedback systems with earphones and acoustic amplifiers.The variables used in the studies included acoustic analysis and self-perceived comfort.The authors found improved voice quality and F0 control in auditory feedback amplification and increased effort in the presence of noise.
Moreover, 25 studies (41.67% of those found) investigated auditory feedback frequency manipulation (pitch-shift); all of them are cross-sectional experimental studies.In general, they aimed to observe the reflex of pitch shifts and its applications in the study samples.Most samples included both sexes (n = 14), having people without vocal complaints (n = 24) and non-occupational voice users (n = 19).The most used tasks were the emission of sustained vowels (n = 18) and singing (n = 3).F0 variation was the variable most studied (n = 19) to assess the effect, though some studies used other variables to understand the reflex, such as the magnitude and direction of the reflex response, electroencephalography, electroglottography, laryngeal imaging, response time, and cepstral measures.All experiments verified reflex responses to pitch shifts in auditory feedback, although each study had different characteristics (Chart 2).

Acoustic measurements of frequency and intensity.
There was an increase in vocal production intensity in both noise conditions.There was also an increase in fundamental frequency (F0), although it was less robust.Participants in each group were instructed to use vocal hygiene strategies, amplifier use or no intervention according to their allocation in the group for six weeks.
Self-perception of voice handicap and severity of the voice problem and perception of the strategy used, acoustic and auditory perceptive analysis.
There were no differences between the sound amplification and vocal hygiene groups.The amplification group reported greater clarity and greater ease of voice production with greater adherence to the proposed strategy.The findings support the clinical utility of sound amplification as an alternative for the rehabilitation of vocal problems in teachers.Reading a text in a soft, normal and loud way, lasting between 1 and 2 minutes, in a semi-reverberant room with and without panels that increase auditory feedback, and in noise conditions of 40 dB and 61 dB.
Self-perception of fatigue, comfort and vocal control.
Participants increased their level of fatigue in the presence of noise and when instructed to speak in a loud style.They lessened fatigue when feedback was increased and when speaking in a smooth style.In self-perception, there was a preference for the normal style without noise.Test the hypothesis that the elimination of kinesthesia would be associated with a greater response to an external auditory disturbance.
Vocalize the vowels /u/ and /i/ with auditory feedback modification by 5 and 1 semitones up and down in conditions with and without vocal fold anesthesia.

Fundamental frequency modifications.
Vocal fold anesthesia increases the response to an externally imposed auditory disturbance.There are differences for these sensory channels: auditory feedback can be used for full F0 control while kinesthesia is used when auditory feedback is not available.
There is a need for more in-depth studies on the multidimensional effects of auditory feedback amplification on the voice.Each study analyzes a type of outcome, but there are few elements to understand the proportions between auditory feedback amplification and decreased voice intensity.
The studies that researched the Lombard effect had different samples, including adult singers, teachers/professors, healthy individuals, reporters, dysphonic individuals, and musicians.Regardless of the population, most results involved increased voice production intensity in the various tasks (23)(24)(25)27,31,33,34,61,62) . Few studies observd feedback suppression in dysphonic subjects.Studies have shown that this population is seemingly more sensitive to the effects of auditory feedback suppression and find it more difficult to return to the habitual speech intensity when the noise is removed (25,31,34,63) .
There is no consensus on the intensity of the masking noise to induce the Lombard effect.Some researchers used signals other than noise via earphones to mask the auditory input, such as music and external noise (26,31,37) .The studies used different intensities, ranging from 40 to 100 dB; the one most used in the methods within this range was 90 dB (29,33,63) .Some studies did not establish a fixed intensity; rather, they were only based on each participant's threshold (23,32) .This shows the need for researching which minimum intensity triggers the Lombard effect and what are the differences between populations, as the literature reports a proportion observed between the noise level and the voice amplification level (18) .
Furthermore, the studies observed that the level of vocal change depends on each person (29,31) , which indicates a difficulty in generalizing it and finding a proportion between noise intensity and voice production intensity.Another interesting finding is that individuals can control the changes caused by auditory feedback suppression, based on simple orders or visual feedback (37,63) .
Another conclusion is that the Lombard effect in beginner singers diminishes tuning precision in complex tasks.Hence, it is inferred that these singers need their hearing to correspond to pitches indicated in complex tasks, whereas more advanced singers do not depend so much on auditory feedback to master their tuning, as previously demonstrated in the literature (25,27) reporters likewise (31,36,62) .
It is hypothesized that training kinesthetic skills helps control voice intensity with less influence from auditory feedback.On the other hand, studies on dysphonic patients have shown that they tend to respond in greater magnitude to the effect (24,30) .
The researchers of studies on pitch-shift effect feedback included variables such as electroencephalography, spectrotemporal dynamics, and functional neuroimaging (41,(45)(46)(47)(48)(49)51) , which verify cortical activities, and associate them between and within hemispheres related to reflex pitch shifts to understand in-depth the response to changes in frequency in auditory feedback (41) . Som studies analyzed neuroimaging and electroencephalography (49) and described that the right hemisphere plays an essential role in pitch modulation.
The tasks varied considerably, including different vowels sustained in constant pitches, corroborating the literature that described the pitch-shift reflex in this task (10,(15)(16)(17) .Some experiments used singing tasks (38,48,54,59) , or the emission of syllables (46,54) , words, and sentences (38,42,44,58) as the frequency is displaced in auditory feedback.The sustained vowels used are not standardized; the most recurrent ones are /a/ and /u/ (17,38,41,43,(45)(46)(47)(49)(50)(51)(52)(53)(54)(55)(56) , though some articles used other ones (38,45,51,54,56) . The stdies have found a correction reflex to the manipulation of the auditory feedback frequencyi.e., if the feedback goes up, participants tend to correct it by decreasing the production frequency, and vice-versa, as described in other articles (15)(16)(17)24) .The literature available describes different results between vocally healthy and dysphonic populations -the latter seems to have a greater reflex response to pitch changes in auditory feedback (42) . However, no quatitative or proportional pattern has been found yet regarding such changes triggered by manipulation among dysphonic and non-dysphonic individuals.Studies have concordantly observed that singers rely more on their internal tuning model than non-singers.The longer the training in singing tasks, the more signers tend to rely on their internal tuning model, rather than on the auditory feedback.The manipulations did not have the same magnitude in trained and untrained singers (47,48,54,57,59) .These data point to the hypothesis that trained individuals have well-established internal models, little influenced by external changes and updates, in contrast with individuals without training or with vocal changes.
The studies also report positive voice production results, with less self-perceived phonatory effort (68,71,74) .However, each experiment used a different method, and even though auditory feedback voice amplification is described as a therapeutic resource that optimizes speech therapy, no research was found addressing its effects along with voice therapy.
Equipment used also varied, and there are no comparisons of the effectiveness obtained with the different forms of using the same type of manipulation.Likewise, few studies observed feedback changes and manipulations in dysphonic individuals.Hence, effectiveness studies are needed to verify the results of using amplification in vocal rehabilitation (66,67,71) .These gaps in the literature are fields of research that still need scientific exploration.
The studies in this scoping review show that the levels of amplification are not standardized and that there is no conclusive value of the decrease in voice production intensity in response to specific values of auditory feedback amplification.Few studies explore intensity proportions or levels in their experiments (61,70) .This gap makes it difficult to establish a contrary relationship or correlation of the proportion found in the Lombard effect (18) .
On the other hand, experiments that associated the effects of auditory feedback suppression and amplification also had different objectives, such as assessing task precision in singers and verifying comfort levels and sound pressure levels in normal individuals (61,(77)(78)(79) .This review did not find standardized levels of amplification or noise insertion, as studies used different experiment methodologies and tasks (singing and text reading tasks).
This scoping review explored experiments and condensed research results with auditory feedback manipulation for adults' speech-motor control.However, the methodological diversity between experiments, sometimes with scarce information, unstandardized speech tasks, different outcome variables, and small samples may have limited the results.Nevertheless, this review pointed out gaps in current knowledge, encouraging further research on the topic, and, therefore, helping increase the knowledge of voice training or therapy.

CONCLUSION
Mapping the current literature on experiments with auditory feedback included in this scoping review shows that different methods are used to amplify, suppress, and manipulate auditory feedback frequency.Results in general are similar regarding the reflex response in voice motor control, observing each experiment's specificities.However, the relationship between the magnitude of manipulation and the responses still needs to be better understood.The main findings in research on auditory feedback for voice motor control demonstrate that, in auditory feedback suppression, individuals tend to increase their voice intensity.In auditory feedback amplification, they decrease voice intensity and have greater control over F0.In frequency manipulations, they tend to correct the manipulation.The few studies that conducted experiments in dysphonic individuals showed that they behave differently from non-dysphonic subjects.

Introduction:
The auditory perception of voice and its production involve auditory feedback, kinesthetic cues and the feedforward system that produce different effects for the voice.The Lombard, Sidetone and Pitch-Shift-Reflex effects are the most studied.The mapping of scientific experiments on changes in auditory feedback for voice motor control makes it possible to examine the existing literature on the phenomenon and may contribute to voice training or therapies.Purpose: To map experiments and research results with manipulation of auditory feedback for voice motor control in adults.Method: Scope review following the Checklist Preferred Reporting Items for Systematic reviews and Meta-Analyses extension (PRISMA-ScR) to answer the question: "What are the investigation methods and main research findings on the manipulation of auditory feedback in voice self-monitoring of adults?".The search protocol was based on the Population, Concept, and Context (PCC) mnemonic strategy, in which the population is adult individuals, the concept is the manipulation of auditory feedback and the context is on motor voice control.Articles were searched in the databases: BVS/Virtual Health Library, MEDLINE/Medical Literature Analysis and Retrieval System online, COCHRANE, CINAHL/Cumulative Index to Nursing and Allied Health Literature, SCOPUS and WEB OF SCIENCE.Results: 60 articles were found, 19 on the Lombard Effect, 25 on the Pitch-shift-reflex effect, 12 on the Sidetone effect and four on the Sidetone/Lombard effect.The studies are in agreement that the insertion of a noise that masks the auditory feedback causes an increase in the individual's speech intensity and that the amplification of the auditory feedback promotes the reduction of the sound pressure level in the voice production.A reflex response to the change in pitch is observed in the auditory feedback, however, with particular characteristics in each study.Conclusion: The material and method of the experiments are different, there are no standardizations in the tasks, the samples are varied and often reduced.The methodological diversity makes it difficult to generalize the results.The main findings of research on auditory feedback on voice motor control confirm that in the suppression of auditory feedback, the individual tends to increase the intensity of the voice.In auditory feedback amplification, the individual decreases the intensity and has greater control over the fundamental frequency, and in frequency manipulations, the individual tends to correct the manipulation.The few studies with dysphonic individuals show that they behave differently from non-dysphonic individuals.

Figure 1 .
Development of the search strategy

Chart 1 .
Studies on manipulations of auditory feedback intensity (Lombard Effect, Sidetone Effect, and Sidetone/Lombard Effect) according to authorship, publication year, study country, objective, method, and conclusions